The technical field of this invention is that of nondestructive evaluation for material characterization, which includes quantitative, model-based assessment of surface, near-surface, and bulk material condition for flat and curved parts or components. Characterization of bulk material condition includes measurement of changes in material state such as degradation or damage, assessment of residual stresses and applied loads, and assessment of processing-related conditions, for example, from shot peening, roll burnishing, or heat treatment. It also includes measurements characterizing the material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, changes in relative position, coating thickness, and coating condition. Each of these includes detection of variations in electromagnetic sensor responses associated with the presence of flaw conditions or microstructural, compositional, or magnetic structure (e.g., domain orientation) changes.
A particular aspect of this invention is related to sensing and monitoring mechanical stress, strain, and load on a material. Stress and load monitoring are important for developing health usage and monitoring systems for vehicles such as rotorcraft. These systems require technologies such as direct load monitoring, on-board damage monitoring and advanced diagnostics for early fault detection to meet the demand for increased safety and reduced operational cost. For example, early detection of damage and cracks in air vehicle structures supports a more effective damage tolerance approach and supplements mechanical diagnostics and usage monitoring. In particular, on-board damage monitoring can provide timely detection of mechanical damages that remain undetectable by conventional methods until the next scheduled inspection, which can enhance safety, improve readiness and mission performance, and reduce maintenance costs. The fields of strain sensing, using strain gages, and load monitoring are relatively mature with numerous approaches that enable monitoring of stresses, strains and loads. These include conventional strain gages, optical fiber strain gages, and ultrasonic methods.
Magnetic field or eddy current sensors have also been used to assess stress on a material. Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity, physical properties (electrical conductivity and magnetic permeability), and geometry (layer thickness) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. As an example, Goldfine et al. have disclosed methods under U.S. Pat. Nos. 5,015,951, RE36,986, 5,453,689, 5,793,206, 6,188,218, 6,252,398, 6,377,039, and 6,657,429 that describe magnetic field sensors that operate in the magnetoquasistatic regime (in other words, the wavelength of traveling waves is long compared to the sensor dimensions). These sensors use precomputed databases of sensor response to estimate the lift-off (sensor proximity) and directional magnetic permeability, directional electrical conductivity, and layer thicknesses for uniform, layered and modified-surface materials. As another example, U.S. Pat. No. 5,828,211 to Scruby et al. describes measuring a response of a directional magnetic field sensor as the sensor is rotated over a test material and using this information to determine the biaxial stress distribution.